The estrus cycle (also œstrus or estrous cycle) refers to the recurring physiologic changes that are induced by reproductive hormones in most mammalian placental females (humans and great apes are the only mammals who undergo a menstrual cycle instead). Estrus cycles start after puberty in sexually mature females and are interrupted by anestrous phases (see below: anestrus). Typically estrus cycles continue until death. Some animals may display bloody vaginal discharge, often mistaken for menstruation.
(http://en.wikipedia.org/wiki/Estrus)